It was in a number of ways a formative period, perhaps the formative period in European history.

ECONOMIST: Medieval history

The task of re-reading Muslim history, specially the earliest years of the post-Prophetic period, is not one of retrospectively reading our standards into what was narrated in the formative period, but finding clues in that narrative to explain the subsequent development of Muslim politics into our time.

CENTERFORSECURITYPOLICY: Muslim-on-Muslim Violence

They spent 10 years in the U.S. during a formative period of their lives, exhibiting normal behavior for first-generation immigrants, said Mitchell Silber, a former intelligence official in the New York Police Department.

WSJ: Homegrown Terror Threat Appears to Be Rising